DOC: stick-table: add missing documentation about gpt0 stored type

The store type 'gpt0' was present in code but was not documented.

The patch fix this and should be backported since 'gpt0' is supported.
[wt: ~1.6-dev4 hence all stable]

doc/configuration.txt

@@ -11182,6 +11182,11 @@ stick-table type {ip | integer | string [len <length>] | binary [len <length>]}
       incremented. Most of the time it will be used to measure the frequency of
       occurrence of certain events (e.g. requests to a specific URL).
 
+    - gpt0 : first General Purpose Tag. It is a positive 32-bit integer
+      integer which may be used for anything. Most of the time it will be used
+      to put a special tag on some entries, for instance to note that a
+      specific behavior was detected and must be known for future matches
+
     - conn_cnt : Connection Count. It is a positive 32-bit integer which counts
       the absolute number of connections received from clients which matched
       this entry. It does not mean the connections were accepted, just that